将索引缩短为 IX_CE_Offering_Status_WaitlistedAt。

将候补迁移改为可重入：先检查列和索引是否存在，再执行 DDL。
附件中已经成功添加 WaitlistedAt、但索引创建失败的数据库可直接重试，不会报重复列。
运行时和 dotnet ef 连接统一开启 AllowUserVariables；Connector/NET 默认关闭该选项。Connector/NET 官方说明
增加所有 MySQL 索引名不得超过 64 字符的回归测试。

src/Jiaowu.Api/Infrastructure/Persistence/AppDbContext.cs

@@ -518,7 +518,7 @@ public sealed class AppDbContext(DbContextOptions<AppDbContext> options)
                 x.CourseSelectionOfferingId,
                 x.Status,
                 x.WaitlistedAt
-            });
+            }).HasDatabaseName("IX_CE_Offering_Status_WaitlistedAt");
             entity.HasOne(x => x.CourseSelectionOffering)
                 .WithMany(x => x.Enrollments)
                 .HasForeignKey(x => x.CourseSelectionOfferingId)

src/Jiaowu.Api/Infrastructure/Persistence/AppDbContextFactory.cs

@@ -12,7 +12,7 @@ public sealed class AppDbContextFactory : IDesignTimeDbContextFactory<AppDbConte
             ?? "Server=localhost;Port=3306;Database=jiaowu;User=__design_time__;Password=__not_used__;";
 
         var options = new DbContextOptionsBuilder<AppDbContext>()
-            .UseMySQL(connectionString)
+            .UseMySQL(MySqlConnectionStringPolicy.ForApplication(connectionString))
             .Options;
 
         return new AppDbContext(options);

src/Jiaowu.Api/Infrastructure/Persistence/DevelopmentSqliteMigrator.cs

@@ -1611,7 +1611,7 @@ public sealed class DevelopmentSqliteMigrator(
         ADD COLUMN "WaitlistedAt" TEXT NULL;
         """,
         """
-        CREATE INDEX IF NOT EXISTS "IX_CourseEnrollments_CourseSelectionOfferingId_Status_WaitlistedAt"
+        CREATE INDEX IF NOT EXISTS "IX_CE_Offering_Status_WaitlistedAt"
         ON "CourseEnrollments" ("CourseSelectionOfferingId", "Status", "WaitlistedAt");
         """
     ];

src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/20260726170000_CourseSelectionWaitlist.cs

@@ -12,26 +12,91 @@ public partial class CourseSelectionWaitlist : Migration
 {
     protected override void Up(MigrationBuilder migrationBuilder)
     {
-        migrationBuilder.AddColumn<DateTime>(
-            name: "WaitlistedAt",
-            table: "CourseEnrollments",
-            type: "datetime(6)",
-            nullable: true);
+        migrationBuilder.Sql("""
+            SET @waitlisted_column_exists = (
+                SELECT COUNT(*)
+                FROM `INFORMATION_SCHEMA`.`COLUMNS`
+                WHERE `TABLE_SCHEMA` = DATABASE()
+                  AND `TABLE_NAME` = 'CourseEnrollments'
+                  AND `COLUMN_NAME` = 'WaitlistedAt'
+            );
+            """);
+        migrationBuilder.Sql("""
+            SET @add_waitlisted_column_sql = IF(
+                @waitlisted_column_exists = 0,
+                'ALTER TABLE `CourseEnrollments` ADD COLUMN `WaitlistedAt` datetime(6) NULL',
+                'SELECT 1'
+            );
+            """);
+        migrationBuilder.Sql(
+            "PREPARE add_waitlisted_column_stmt FROM @add_waitlisted_column_sql;");
+        migrationBuilder.Sql("EXECUTE add_waitlisted_column_stmt;");
+        migrationBuilder.Sql("DEALLOCATE PREPARE add_waitlisted_column_stmt;");
 
-        migrationBuilder.CreateIndex(
-            name: "IX_CourseEnrollments_CourseSelectionOfferingId_Status_WaitlistedAt",
-            table: "CourseEnrollments",
-            columns: new[] { "CourseSelectionOfferingId", "Status", "WaitlistedAt" });
+        migrationBuilder.Sql("""
+            SET @waitlist_index_exists = (
+                SELECT COUNT(*)
+                FROM `INFORMATION_SCHEMA`.`STATISTICS`
+                WHERE `TABLE_SCHEMA` = DATABASE()
+                  AND `TABLE_NAME` = 'CourseEnrollments'
+                  AND `INDEX_NAME` = 'IX_CE_Offering_Status_WaitlistedAt'
+            );
+            """);
+        migrationBuilder.Sql("""
+            SET @add_waitlist_index_sql = IF(
+                @waitlist_index_exists = 0,
+                'CREATE INDEX `IX_CE_Offering_Status_WaitlistedAt` ON `CourseEnrollments` (`CourseSelectionOfferingId`, `Status`, `WaitlistedAt`)',
+                'SELECT 1'
+            );
+            """);
+        migrationBuilder.Sql(
+            "PREPARE add_waitlist_index_stmt FROM @add_waitlist_index_sql;");
+        migrationBuilder.Sql("EXECUTE add_waitlist_index_stmt;");
+        migrationBuilder.Sql("DEALLOCATE PREPARE add_waitlist_index_stmt;");
     }
 
     protected override void Down(MigrationBuilder migrationBuilder)
     {
-        migrationBuilder.DropIndex(
-            name: "IX_CourseEnrollments_CourseSelectionOfferingId_Status_WaitlistedAt",
-            table: "CourseEnrollments");
+        migrationBuilder.Sql("""
+            SET @waitlist_index_exists = (
+                SELECT COUNT(*)
+                FROM `INFORMATION_SCHEMA`.`STATISTICS`
+                WHERE `TABLE_SCHEMA` = DATABASE()
+                  AND `TABLE_NAME` = 'CourseEnrollments'
+                  AND `INDEX_NAME` = 'IX_CE_Offering_Status_WaitlistedAt'
+            );
+            """);
+        migrationBuilder.Sql("""
+            SET @drop_waitlist_index_sql = IF(
+                @waitlist_index_exists > 0,
+                'DROP INDEX `IX_CE_Offering_Status_WaitlistedAt` ON `CourseEnrollments`',
+                'SELECT 1'
+            );
+            """);
+        migrationBuilder.Sql(
+            "PREPARE drop_waitlist_index_stmt FROM @drop_waitlist_index_sql;");
+        migrationBuilder.Sql("EXECUTE drop_waitlist_index_stmt;");
+        migrationBuilder.Sql("DEALLOCATE PREPARE drop_waitlist_index_stmt;");
 
-        migrationBuilder.DropColumn(
-            name: "WaitlistedAt",
-            table: "CourseEnrollments");
+        migrationBuilder.Sql("""
+            SET @waitlisted_column_exists = (
+                SELECT COUNT(*)
+                FROM `INFORMATION_SCHEMA`.`COLUMNS`
+                WHERE `TABLE_SCHEMA` = DATABASE()
+                  AND `TABLE_NAME` = 'CourseEnrollments'
+                  AND `COLUMN_NAME` = 'WaitlistedAt'
+            );
+            """);
+        migrationBuilder.Sql("""
+            SET @drop_waitlisted_column_sql = IF(
+                @waitlisted_column_exists > 0,
+                'ALTER TABLE `CourseEnrollments` DROP COLUMN `WaitlistedAt`',
+                'SELECT 1'
+            );
+            """);
+        migrationBuilder.Sql(
+            "PREPARE drop_waitlisted_column_stmt FROM @drop_waitlisted_column_sql;");
+        migrationBuilder.Sql("EXECUTE drop_waitlisted_column_stmt;");
+        migrationBuilder.Sql("DEALLOCATE PREPARE drop_waitlisted_column_stmt;");
     }
 }

src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/20260726180000_CourseSelectionGradeLimits.Designer.cs

@@ -748,7 +748,8 @@ namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql
 
                     b.HasIndex("StudentId", "Status");
 
-                    b.HasIndex("CourseSelectionOfferingId", "Status", "WaitlistedAt");
+                    b.HasIndex("CourseSelectionOfferingId", "Status", "WaitlistedAt")
+                        .HasDatabaseName("IX_CE_Offering_Status_WaitlistedAt");
 
                     b.ToTable("CourseEnrollments");
                 });

src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/AppDbContextModelSnapshot.cs

@@ -745,7 +745,8 @@ namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql
 
                     b.HasIndex("StudentId", "Status");
 
-                    b.HasIndex("CourseSelectionOfferingId", "Status", "WaitlistedAt");
+                    b.HasIndex("CourseSelectionOfferingId", "Status", "WaitlistedAt")
+                        .HasDatabaseName("IX_CE_Offering_Status_WaitlistedAt");
 
                     b.ToTable("CourseEnrollments");
                 });

src/Jiaowu.Api/Infrastructure/Persistence/MySqlConnectionStringPolicy.cs

@@ -0,0 +1,15 @@
+using MySql.Data.MySqlClient;
+
+namespace Jiaowu.Api.Infrastructure.Persistence;
+
+public static class MySqlConnectionStringPolicy
+{
+    public static string ForApplication(string connectionString)
+    {
+        var builder = new MySqlConnectionStringBuilder(connectionString)
+        {
+            AllowUserVariables = true
+        };
+        return builder.ConnectionString;
+    }
+}

src/Jiaowu.Api/Program.cs

@@ -141,14 +141,16 @@ builder.Services.AddDbContextPool<AppDbContext>(options =>
         throw new InvalidOperationException(
             "缺少 MySQL 连接串。请通过 ConnectionStrings__MySql 环境变量配置。");
     }
-    options.UseMySQL(connectionString, mySqlOptions =>
-    {
-        mySqlOptions.CommandTimeout(databaseOptions.CommandTimeoutSeconds);
-        mySqlOptions.EnableRetryOnFailure(
-            maxRetryCount: 5,
-            maxRetryDelay: TimeSpan.FromSeconds(10),
-            errorNumbersToAdd: null);
-    });
+    options.UseMySQL(
+        MySqlConnectionStringPolicy.ForApplication(connectionString),
+        mySqlOptions =>
+        {
+            mySqlOptions.CommandTimeout(databaseOptions.CommandTimeoutSeconds);
+            mySqlOptions.EnableRetryOnFailure(
+                maxRetryCount: 5,
+                maxRetryDelay: TimeSpan.FromSeconds(10),
+                errorNumbersToAdd: null);
+        });
 });
 
 var redisConnectionString = builder.Configuration.GetConnectionString("Redis");